Lesson Plan

Info

Cell Replication

Students explore the structure and function of cells and mitosis.

Lesson Summary

Overview

In this activity, students explore the structure and function of cells. They learn that all organisms are made up of one or more cells and that single-celled organisms do many of the things multicellular organisms do: grow, eat, excrete, and reproduce. Students observe that all the things cells need to take in or get rid of pass through the cell membrane. Using balloons as models, they explore the relationship between the surface area of a cell and the cell's volume, and discover that the ratio of surface area to volume constrains cell size. Finally, students learn about cell replication, by which a cell makes a copy of its genetic material and then divides into two "daughter" cells, each identical to the parent cell.

Objectives

Recognize that all organisms are made up of one or more cells

Observe how the cell membrane serves as gatekeeper for all substances that enter and leave the cell

Understand that as a cell grows larger, its surface-area-to-volume ratio decreases

Learn that, prior to dividing, cells must replicate their DNA

Discover that cell division is the means by which single-celled organisms reproduce and multicellular organisms grow

Observe that mitosis results in two daughter cells that are identical to the parent cell

Materials

Before the Lesson

Review the concepts of cell structure and function, using the lesson App Exception: tdc02.sci.life.cell.lp_strufx Make sure students understand the gatekeeper role of the cell membrane.

Read the background essay that accompanies each resource.

After the Lesson

Do an experiment with students to demonstrate how a cell's surface-area-to-volume ratio affects the rate of diffusion of substances into and out of the cell. (See BSCS Biological Science: An Ecological Approach for a possible lab.)

What are some of the waste products that cells must eliminate to survive?

Name some of the systems in a multicellular organism that serve to take in and distribute substances and get rid of wastes.

Do single-celled organisms have transport systems similar to those of multicellular organisms?

3. Explain to students that single-celled organisms take in and release substances by means of diffusion across the cell membrane, and that this process limits cell size. Ask students to consider why this might be.

4. Blow up a balloon and tie it off. Tell students that the balloon represents a cell. Then blow up a second balloon larger than the first and tie it off. Hold up the balloons and ask students:

As a cell grows, what happens to its surface area?

As a cell grows, what happens to its volume (the amount of space the cell takes up)?

What is the relationship between the surface area and the volume of a cell? (As one increases [or decreases], so does the other.)

If the surface area of a cell doubles, does the volume double also? (No. For example, a balloon with a surface area of 300 square inches has a volume of about 490 cubic inches. If the surface area doubles to 600 square inches, the volume increases to nearly 1400 cubic inches -- almost three times the original volume. The surface area and volume change disproportionately to one another.

5. Now have students work in pairs to demonstrate this for themselves. Provide each pair with two balloons and ask them to blow up and tie off the balloons. One balloon should be noticeably larger than the other. Have pairs of students measure the radii of the two balloons and calculate the surface area and volume of each. (Note: Tell students to assume that each balloon is a perfect sphere. The formula for calculating the surface area of a sphere is A = 4 π r 2. The formula for calculating the volume of a sphere is V = 4/3 π r3.)

Now remind students that cells rely on diffusion to take in what they need and to get rid of wastes. This means that it is best to have all cell material as close to the cell membrane as possible. Ask students:

What happens to the surface-area-to-volume ratio as a cell gets larger?

How would a smaller ratio (i.e., a smaller surface area per volume) affect the cell's ability to function?

Is it easier for a large cell or for a small cell to take in and get rid of substances by diffusion? Why?

Part II: DNA Replication and Cell Division

What is the difference, if any, between the two daughter cells that result from cell division following mitosis?

7. Explain that single-celled organisms reproduce via DNA replication and cell division. When single-celled organisms divide, the size of their population increases and they pass their genetic material on to the next generation. This is called asexual reproduction, and it produces daughter cells that are identical to the parent cell.

8. Explain that multicellular organisms use DNA replication and cell division to grow by increasing the number of cells they have and to repair their tissues by replacing damaged cells. Tell students that in the time it took them to watch the Mitosis video each of them lost approximately 40,000 skin cells. Ask:

How does the body compensate for this high rate of skin cell loss? (The body regenerates cells at a rate equal to the rate of skin loss, thus our skin is maintained.)

Optional Activity

Part III

9. Ask:

What would happen if cell replication within a multicellular organism proceeded in an uncontrolled manner?

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